Composites in Light-Sport Aircraft

Anticipating early approval, manufacturers of composite aircraft were lining up in early 2003 to launch lightweight recreational flying machines that conformed to proposed rules for the U.S. Federal Aviation Admin.'s (FAA) Light-Sport Aircraft (LSA) category. Proposed in 2002, the LSA category took a bit longer than even the FAA expected to make its official debut. It was well worth the wait: As of September 2004, aircraft in the new category could have a maximum of two occupants in an unpressurized cabin, and are limited to a single piston- or rotary-type engine (no turbines), a fixed-pitch or ground-adjustable propeller (no variable pitch or constant speed units) and fixed landing gear. LSAs must display an "N" number (registration number) and are allowed a maximum gross take-off weight (GTW) is 599 kg/1,320 lb (649 kg/1,430 lb for water operation). Maximum speed in level flight is 120 knots/138 mph and maximum stall speed must not exceed 45 knots/51 mph (calibrated airspeed in landing configuration, with flaps/slats/brakes deployed). (Note: FAA is considering raising the GTW to 680 kg/1,500 lb and lifting the turbine ban.)

LSAs may be produced in kit form, and aircraft previously available only in kits may now be sold as finished aircraft, a boon to general aviation manufacturers (kit planes are classed as Experimental LSAs or ELSAs, while fully-finished, factory-built LSAs are denoted Special LSAs or SLSAs). Type certification is not required. Instead, manufacturers must go through a much less expensive testing regimen to achieve an LSA airworthiness certificate. Additionally, FAR (Federal Aviation Regulation) Part 103 Ultralights can be licensed as LSAs and aircraft in other FAA categories may be relicensed as LSAs if they meet the basic limitations.

Significantly, ASTM International was brought into the LSA working group to harmonize FAA's proposed regulations with those of the Joint Aviation Resolution - Very Light Aircraft (JAR-VLA) protocol that provides the basis for rules governing the light aircraft elsewhere in the world. The result is an LSA rule that leaves an astounding amount of latitude for designers and a global marketplace in which to sell the result.

In the year since LSA rules took effect, a number of manufacturers have fielded composite-intensive aircraft. The following is a representative sampling of those not included in HPC's previous coverage.

Eastern European imports

Several European aircraft manufacturers have built or adapted planes for the U.S. market and now sell them through domestic distributors, including three from nations formerly in the Soviet bloc.

One of the earliest was the StingSport, a low-wing, tricycle-gear sport plane built by TL Ultralight sro (Kralvoe, Czech Republic) and imported and distributed by SportairUSA LLC (Little Rock, Ark.). An LSA-certified stablemate to its predecessor, the StarSport, the StingSport seems identical at first glance. The fuselage components for the two are, in fact, pulled from the same molds. Typical of light planes built for Europe, where travel distances are generally much shorter and fuel, by comparison, is grievously expensive, both seat two, and have the small engine and gas tank befitting LSA requirements. (More seats equal more weight equals more fuel usage, so the "family plane" never caught on the way it did in the U.S.)

But a closer inspection reveals much that is different. The StarSport was clearly designed with European pilots in mind. Its constant-chord, or "Hershey bar" wing planform spans 8.4m/27.7 ft, with a wing area of 12.1m2/130.2 ft2 a maximum take-off weight of 1,320 lb and a resulting wing loading of just over 10 psf, it takes off at just 42 knots/48 mph. This means it can operate from unimproved grass or dirt strips, as long as they're not too rough. By contrast, the StingSport has a tapered wing, which, with the same wingspan, provides about 2.3m2/25 ft2 less wing area (10.8m2/116.4 ft2). Even with a 80-hp version of the Rotax 912, a widely used Ultralight powerplant, and a standard three-blade, ground-adjustable propeller, the StingSport gives up a little to its stablemate in both takeoff and stall speeds. According to Sportair, the wings are removable in as little as 15 minutes, and lock together through the fuselage, in a manner similar to sailplanes.

The StingSport also substitutes a fixed stabilizer with conventional elevator for the StarSport's flying tail or "stabilator," and has conformal cowling in place of the latter's pressure cowling. Conformal cowlings follow the engine contours much more closely. Rather than a large plenum sitting atop the engine, cooling air is directed to exactly the areas where it is needed. This permits the use of smaller inlets, presents less frontal and wetted area and ultimately reduces drag.

But the biggest difference is beneath the paint: The StingSport replaces the StarSport's glass-reinforced epoxy structures with carbon composites. Most large panels, such as the wing surfaces and fuselage halves, are made from sandwich construction of carbon/epoxy prepreg from Hexcel (Dublin, Calif.) cored with Divinycell foam from DIAB Inc. (Desoto, Texas) and assembled with epoxy adhesives supplied by MGS Kunstharzprodukte GmbH (Stuttgart, Germany, a div. of Bakelite AG, which is now part of Hexion Specialty Chemicals). Wing and fuselage skins are bagged and oven-cured, while more heavily loaded components, such as the wing spar caps, are autoclave-cured. (Hexcel also is the source for the glass/epoxy prepregs used on the StarSport.) The shear web, being at the neutral axis, has an orientation of ±45°, while the caps are unidirectional tape, running axially (0°).

The StingSport has no gel coat finish. The demolded carbon parts are joined and finished in white paint (the manufacturer notes that some very minor carbon print-though is to be expected), with colors added through the use of self-adhesive graphic films. Elimination of gel coat cuts about 22.7 kg/50 lb from the overall weight, helping to achieve its 345 kg/760 lb empty weight.

Another entry from the Czech Republic is a distinct departure from the StingSport's conventional design. The UFM-13 Lambada from Urban Air sro (Libchavy, Czech Republic) is all about the wing. Two different removable wingtips are available for the plane's mid-wing configuration: a semi-elliptical tip, which provides nearly 12m/39 ft of span; and a highly tapered, somewhat upturned tip, which increases wingspan to 13.7m/45 ft. In the longer configuration, the wing area climbs to almost 10.7m2/115 ft2. The long wing has an impressive 30:1 glide ratio (a comparison of wing lift to drag). Equipped with the same basic powerplant as the StingSport (a Rotax 912), the Lambada's larger wing slows cruising speed to about 81 knots/93 mph. But what it gives up in cross-country utility, it gains in glide. If its engine were shut down at 3,048m/10,000-ft altitude, its pilot could travel almost 60 miles before touchdown. It's a true soaring machine.

Prototyped in 1996 and load tested and flight tested at Brno University's Aeronautical Engineering Institute (Brno, Czech Republic), the Lambada has a conventional tractor design, with a T-tail, side-by-side seating and a choice of tricycle or taildragger landing gear.

The Lambada has a smaller sibling, the UFM-10 Samba. Available with the same power options, the plane has a higher cruising speed of 108 knots/124 mph. but still has a respectable 19:1 glide ratio. The Samba has a conventional tail, as opposed to the Lambada's T-tail. A newer variant, the Samba XXL, differs mainly in that the wing is much smaller, with a 9.1m/30 ft span and an area of about 7.4m2/80 ft2. This, in turn, means that it looks and flies less like a sailplane and more like an airplane. The plane's glide ratio is thus reduced to 18:1, but with an empty weight of 265 kg/584 lb and a maxi-mum take- off weight of only 472.5 kg/ 1,042 lb, its cruise speed jumps to 118 knots/135 mph, to make it useful for some fairly serious traveling. It also has a roomier 1.15m/3.75-ft wide cabin (compared to 1.06m/3.5-ft on the UFM-10).

All three aircraft are made by nearly identical methods. Glass/epoxy prepreg is layed up in female molds and oven cured. Ply schedules were tested via finite element analysis, using a 3-D model built with MSC.Nastran, from MSC.Software (Santa Ana, Calif.). The wingskins feature a sandwich con-struction shell over a beam with spar caps reinforced with unidirectional carbon fiber, permitting strut-free operation and simplifying wing removal for ground transport. When wings are reattached, the spars are designed to interlock with great precision and are held in position with a single steel locking pin. The fuselage is a monocoque with a high-gloss finish to provide optimum strength-to-weight and low drag. Parts consolidation is taken seriously — even the seat frames are integral to the fuselage.

A pair of LSA-compliant aircraft, the Sinus and the Virus, are available from light aircraft manufacturer Pipistrel (Ajdovscina, Slovenia). Like Urban Air's Lambada, the Sinus has a glider-like span of 15m/49 ft, with semi-elliptical tips culminating in winglets canted up at about 45°. A two-seat, high-wing motor-glider, it comes with two power options. The Sinus 503 (so named for its powerplant, a 50-hp, two-stroke Rotax 503) has sufficient power to handle the plane's light airframe (265 kg/584 lb empty weight and a maximum takeoff weight of 450 kg/992 lb). The Sinus 912 is the same airplane, but with the 80-hp version of the four-stroke Rotax 912. The published performance figures for both aircraft are interesting when compared: The max level speed for the 503 is100.5 knots/115.6 mph, while the 912 clocks 129.6 knots/149 mph (a bit too high for LSA compliance). The additional horsepower provides a shorter take-off distance and high rate of climb for owners dealing with short, rough landing strips, especially where ambient temperature and elevation is high. For this reason, and because of the low LSA-required speeds, flaps and control surfaces are generous in size, with quite a bit of deflection.

The Virus differs from the Sinus primarily in the wing configuration. The Virus is more conventional with a span of 12.4m/38 ft (area of 11m2/118.4 ft2), featuring a tapered planform with downturned tips. Despite the difference, cruise speeds are very close; 119 knots/137 mph for the Sinus 912, and 121.5 knots/140 mph for the Virus, equipped with the same engine. Similarly, the glide ratios are close; 27:1 for the Sinus, 24:1 for the Virus. The Virus is slightly heavier, but its maximum take-off weight is identical to the Sinus. Both have T-tails. The Sinus has a tailwheel, but the Virus' landing gear is of the tricycle configuration.

The fuselages for both aircraft are constructed of glass fiber/epoxy prepreg, with aramid or carbon in critical areas. During layup, doublers serve to reinforce the control surface hinge points and the attach points for the linkage. The planes' designer, Ivo Boscarol, says that the wing is a sandwich construction, with face sheets primarily of glass/epoxy over foam core, with the carbon/epoxy spar caps of autoclave-cured uni tape. The prepregs were obtained from P-D Interglas Technologies (Erbach, Germany).

Italian stallion

S.G. Aviation Inc. (Bolton, Ontario, Canada) imports the Rally 105, a high-wing aircraft of conventional layout, built by SG Aviation Industria Aeronautica Italiana (Saubadia, Italy). The plane is available fully assembled or in kit form, in each case with either an aluminum wing (Ultralight version) or composite wing (Light-Sport version). This little airplane actually debuted in the early '90s. Because the company's other aircraft (a low-wing aluminum model, and a composite amphibian) were better sellers, the Rally was discontinued in 1993. With the advent of the LSA category, the decision was made to bring it back.

According to designer Giovanni Saledo, the resurrected Rally features mostly small design changes, with the exception of the fuselage, which is now made entirely of triaxially woven carbon in a solid laminate rather than the typical foam- or honeycomb-cored sandwich. The result is a very light, stiff structure, because the flexural and torsional loads can be picked up without a lot of extraneous fiber going along for the ride.

The composite wing skin is a sandwich of foam core between glass triaxial prepreg skins. The prepreg, from Axson (Saint-Ouen-l'Aumone, France), features glass fiber from G. Angeloni srl (Quatro d'Altino, Italy) and resin from Huntsman Advanced Materials (Basel, Switzerland). The wings have generously wide 102 mm/4-inch spar caps. Saledo says the width ensures that a kit builder who gets careless with the bonding process is unlikely to leave a catastrophic defect. The skins are bagged and oven cured over foam cores.

The Rally's wing options have identical wingspans (9.18m/30.1 ft) and wing areas (12.48m2/134.3 ft2). The all-composite aircraft has an empty weight of 290 kg/638 lb and GTW is only 450 kg/990 lb. It meets LSA criteria in every other respect, but with a 100-hp engine on tap, it will do 129.5 knots/149 mph, 11 mph over the LSA limit. S.G. Aviation is still in discussion with the FAA to determine a way to bring the plane into compliance.

Made in the U.S.A.

Vulcan AeroForge LLC d/b/a Ion Aircraft (St. Paul, Minn.) will soon bring to market an LSA with a design — and history — that are unusual. Back in the '90s, at the EAA AirVenture event in Oshkosh, Wis., a Kansas company called DreamWings previewed a yet-to-be-built two-seater called the Valkyrie. Customers lined up, checkbooks in hand, to make deposits. The plane had a lot of appeal — a sleek, slender tandem fuselage, a raised rear seat for great visibility, a high, twin-boom tail, and a pusher power-plant tucked between the tail booms. Alas, the company tried to amortize its front-end costs with deposit money, a mistake that has killed many programs. In 2001, the venture failed and its 145 customers lost a total of $1.5 million in deposits. But a group within this customer group acquired the rights to the design. Steve Schultz, president of the group and an engineer, led fellow customers through a series of design iterations.

Early on, filament winding was considered for the tapered cylindrical tail booms, but was determined to be a less-than-optimum choice for a flexural beam. Instead, the Ion group went with left and right shells, fabricated with a carbon uni-tape layup, bagged and autoclave-cured. Likewise the vertical fins and horizontal stabilizer, with a generous radius where the two meet.

An aramid/carbon hybrid was considered for the fuselage but Schultz notes that the CTE mismatches posed challenges, especially in highly stressed areas. When heated, aramid tends to shrink slightly, while carbon experiences a very small expansion. Therefore, when parts are molded using a 350-cure epoxy system, warp and springback are risks, particularly in processes where parts receive a freestanding postcure. For these reasons, Ion went with an all-carbon laminate.

At first, an all-carbon wing was considered. Since it would be detach-able, however, there were concerns about the possibility of undetectable laminate damage during removal and transport. Ion went with a composite center-wing section but with removable outer sections of aircraft designer and manufacturer Jim Bede's well-known tubular spar bonded aluminum design (BedeCorp. LLC, Medina, Ohio). Its I-beam spar bolts to the center section much like the wings of many sailplanes.

At this point, the aircraft is almost entirely carbon fiber/epoxy, except for the bonded aluminum outer wings. The nonstructural cowl on the rear-facing engine is fiberglass. Since pusher designs present greater cooling challenges, cooling inlets will be "super-sized," but as a precaution, they'll go with a matrix that will tolerate a bit more heat, such as a cyanate ester or a bismaleimide.

Ion presented a prototype at this year's EAA AirVenture show. In production, the plane will be available as the Ion 100, in ELSA-compliant kit form. Two wing lengths will be available: A longer version, called the "loiter wing," makes the plane LSA-compliant and provides greater endurance due to its greater area and higher aspect ratio. It also provides more total lift, lighter wing loading and lower induced drag. A shorter wing option permits higher air speeds, but makes the plane a little too fast for the LSA rules. Also on the way is an SLSA, called the Ion 120.

Taking flight

Though the LSA market is flourishing, Bill Canino, owner of SportairUSA, notes that the Airbus A-380 and Boeing 787 are already starting to squeeze supplies of aerospace-grade carbon fiber. Nobody knows with certainty what shape the supply will take in the next year or even the next few months. But it's clear that FAA's LSA category is helping to revitalize general aviation

and advance the incorporation of composites into light aircraft. The SP/LSA initiative is showing the potential to become every bit the breakthrough it was intended to be.

Author's Note: Many thanks to Robert D. Wood (U.S. Sport Aviation Expo) and the EAA staff for invaluable assistance in the preparation of this article.